The invention relates to polyisocyanurate plastics, to a process for producing the polyisocyanurate plastics of the invention, and to the use thereof for producing coatings, films, semi-finished products or mouldings.
Polymers having polyisocyanurate structure are known for their high thermal and flame stability. Polyisocyanurate-containing foams based on aromatic diphenylmethane 4,4′-diisocyanate (MDI) and polyetherpolyols and polyepoxides are widespread particularly as high-performance insulating materials for example on account of their low heat conductivity. See C. E. Schildknecht and I. Skeist, Polymerization Processes, pp. 665-667, Wiley, New York (1977). However, MDI-polyisocyanurate-containing foams, as is generally known of aromatic polyurethanes, have only low light stability and a tendency towards severe yellowing.
There has therefore been no lack of attempts to synthesize polyisocyanurate plastics based on aliphatic light-resistant isocyanates.
For example, European Polymer Journal, vol. 16, 147-148 (1980) describes the catalytic trimerization of monomeric 1,6-diisocyanatohexane (HDI) at 40° C. to give a clear transparent polyisocyanurate plastic free of isocyanate groups. For this, however, 15% by weight of dibutyltin dimethoxide are required as trimerization catalyst, and this has a severe negative impact on the thermal stability and colour stability of the products. European Polymer Journal, Vol. 16, 831-833 (1980) describes the complete trimerization of monomeric HDI to give a polyisocyanurate at a temperature of 140° C. using 6% by weight of tributyltin oxide as catalyst.
The thesis by Theo Flipsen: “Design, synthesis and properties of new materials based on densely crosslinked polymers for polymer optical fiber and amplifier applications”, Rijksuniversiteit Groningen, 2000 describes the trimerization of monomeric HDI with a neodymium/crown ether complex as catalyst. The polyisocyanurate obtained, which is said to have good optical, thermal and mechanical properties, was studied in the context of the thesis for its suitability for optical applications, especially as polymeric optical fibres. Flipsen gives a detailed description of the prerequisites for clear non-yellowed polyisocyanurates. Explicit mention should be made here of avoidance of impurities, water, dimers, high catalyst concentration and high temperatures at the start of the reaction. Troublesome side reactions are the reaction with water to give ureas, and of uretdiones to give carbodiimides with blister formation. According to Flipsen, high transparency polyisocyanurates with a Tg of 140° C. are obtained only under ideal conditions with a soluble neodymium/crown ether catalyst and a prereaction at room temperature or 60° C. or room temperature and post reaction at temperatures of up to 140° C. over a long period of more than 24 h. A disadvantage of the described process is that it is a time-consuming multistage process with complicated reaction regime, the large-scale conversion of which is problematic.
The subject of Moritsugu, M., Sudo, A. and Endo, T., J. Polym. Sci. A Polym. Chem. 2013, 51, 2631-2637, is the polymerization of monomeric HDI and MDI with Na-paratolyl sulphonate in various ratios in DMI solution at 150° C. Primarily, clear and colourless products are obtained which, following the reaction, have to be powdered and be purified by means of Soxhlet extraction of solvents and unreacted constituents. In the case of pure HDI, a conversion of 94% is obtained following extraction of 6% soluble fractions. The polyisocyanurate has a Tg of 115° C. The trimerization is conducted in amounts of approx. 1 g in the 25 ml flask. The process thus described is technically problematic on account of the multistage process and the high fraction of extractable components.
The production of polyisocyanurates is described in the prior art primarily starting from liquid monomeric diisocyanates (e.g. stearyl diisocyanate, dodecyl diisocyanate, decyl diisocyanate, nonyl diisocyanate, octyl diisocyanate, HDI, BDI, PDI, IPDI, H12MDI, TDI, MDI, NDI, NBDI), of aliphatic and aromatic nature. The heat tonality of the trimerization reaction to polyisocyanurates is so high (−75 kJ/mol NCO) that a reaction starting from monomeric diisocyanates, particularly in the case of monomeric diisocyanates with high isocyanate content (e.g. BDI, PDI, HDI, TIN), can typically not be conducted on an industrial scale and under adiabatic conditions as typically arise in the inside of volume bodies during strongly exothermic polymerization processes, but only in small quantitative amounts under strict temperature control.
“Adiabatic conditions” means here in particular that a complete dissipation of the heat of reaction released during the exothermic reaction to the surrounding area is not possible. Thus, typically no homogeneous conditions can be realised in volume bodies and adiabatic conditions prevail particularly in the inside of the volume bodies which can lead, in the case of an exothermic reaction, to a considerable local temperature increase. These local hotspots are extremely critical if it is a question of producing functionally homogeneous products.
A further problem is that aromatic monomeric diisocyanates and many arylaromatic or alicyclic monomeric diisocyanates can be homo- and co-trimerized only to low conversions. Often plasticizing or co-dissolving reactants have to be added. Otherwise, the reaction freezes at high residual isocyanate contents and typically opaque and discoloured products are obtained. The use of plasticizing and co-dissolving reactants is again disadvantageous since these lead to less chemically and thermally inert structural elements such as allophanates, ureas, urethanes, thiourethanes and oxazolidinones, polyesters, polyethers, and at high temperatures to uretdiones with subsequent carbodiimidation and carbon dioxide elimination, and to asymmetric isocyanates. The production of polyisocyanurates having largely exclusively isocyanurate structures as structural element is therefore not possible.
Temperature control during the production of highly reacted polyisocyanurates is of enormous importance since, on account of the high isocyanate contents of the monomeric starting materials under adiabatic conditions, as typically prevail during trimerizations in volume bodies, on account of the exothermic reaction, temperatures of more than 300° C. can arise which may lead to the direct decomposition of the products and even to the in situ evaporation of the monomeric diisocyanates. Besides the occupational hygiene disadvantages due to the released toxic monomeric diisocyanates or decomposition products, the formation of blisters is very troublesome here.
Consequently, polyisocyanurates have hitherto usually only found practical applications as crosslinking agents in paint chemistry, the production of which involves stopping the trimerization reaction at low conversions and removing excess unreacted monomeric diisocyanate. Thus, DE 31 00 263; GB 952 931, GB 966 338; U.S. Pat. Nos. 3,211,703, 3,330,828 envisage conducting the reaction either in dilution or only up to low conversion values with very precise temperature control during the production of crosslinking agents based on isocyanurates starting from aliphatic and mixed aliphatic and aromatic monomeric diisocyanates. There is deliberately no formation here of crosslinked polyisocyanurate plastics, but only of oligomeric, soluble products of low viscosity.
A common feature of the aforementioned processes is that the trimerization is started at low temperatures. Higher trimerization temperatures, particularly at the start of the trimerization, can be controlled only with difficulty starting from monomeric diisocyanates and lead to considerable side reactions in the form of uretdiones, carbodiimides, and also to blistering and discoloration.
Another common feature of the described processes is that they are unsuitable for obtaining highly converted polyisocyanurates with a low residual content of free isocyanate groups in efficient industrial processes, especially those which are largely free from extractable monomers. Nor is it possible in this way, by the processes known from the prior art, to effect trimerization at elevated temperatures in open reaction vessels without risking significant release of monomeric diisocyanates into the environment.
WO 2015/166983 discloses the use of isocyanurate polymers for encapsulating LEDs. The use of carboxylates and alkoxides of alkali metals, alkaline earth metals or zirconium, in combination with complexing agents such as crown ethers or polyethylene glycols or polypropylene glycols, and organic tin compounds is not disclosed. Moreover, there is no particular disclosure for coating of specific substrates.
U.S. Pat. No. 6,133,397 discloses coatings made by trimerizing oligomeric polyisocyanates. However, the curing temperatures disclosed are significantly above room temperature.